Returning to the Fringe after the success of last year's Women's Hour, Rebecca Biscuit and Louise Mothersole turn their anarchic performance practice to the housing crisis in their latest effort, Letters to Windsor House. With bone-dry humour and a twisted whimsy, Sh!t Theatre use an apparently frivolous passion project as the foundation for a biting critique of unfairness and injustice in modern Britain.

Tired of letters to previous tenants taking up room on the 'Sh!t Theatre Awards Shelf', Becca and Louise, tenants of the grandly named Windsor House, a run-down London council flat, set about excavating the ever-growing pile. From the bills, bank statements, and breast cancer leaflets they uncover, the pair piece together the lives of their predecessors, making canny use of a loop pedal to soundtrack their findings and fantasies with irrepressibly catchy songs.

Beneath their musical hijinks, however, a righteous fury bubbles. Dancing impishly along the line of legality, Sh!t Theatre's playful practice constructs an implicit but unmistakable critique of austerity Britain, poking holes in a political and economic framework that permits widespread housing shortages and endemic social degradation. Their indignant political agitation, disguised by a whimsically playful approach, is all the more effective for its subtlety.

At certain points, the show appears to struggle to fill its running time, and some repeated elements do little more than create the sense that it is treading water. But at its focused best, the production illustrates the considerable theatrical expertise of its makers. Playful, anarchic, and quietly seditious, Letters to Windsor House is a finely tuned political polemic.